<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script src="../js/vue.js"></script>
    <div id ="app">
        <p>{{firstName}}</p>
        <p>{{lastName}}</p>
    </div>
    
    <script>
        let app = new Vue({
            el:"#app",
            data:{ 
                firstName:'Foo',
                lastName:'Bar',
                // fullName:'Foo Bar'
            },
            watch:{
                firstName:function(val){
                    this.fullName = val+''+this.lastName;
                },
                lastName:function(val){
                    this.fullName = this.firstName+''+val;
                }
            },
            computed:{
                fullName:{
                    get:function(){
                        return this.firstName+''+this.lastName;
                    },
                    set:function(newValue){
                        var names = newValue.split('');
                        this.firstName = names[0];
                        this.lastName = names[names.length-1];
                    }
                }
            }
        })
    </script>
</body>
</html>